The production of gold, copper and nickel increasingly relies on the treatment of ore bodies which contain significant quantities of arsenic-bearing minerals. In some cases, the ores and concentrates recovered from these ore bodies contain levels of arsenic which are too high to be accepted by most smelters, and therefore arsenic must typically be removed prior to smelting. Also, arsenic in many gold ores must be removed in order to prevent gold encapsulation by iron arsenates during the oxidative roast used to remove sulfur and organic carbon from refractory ores.
Arsenic removal is typically carried out by subjecting an arsenic-bearing ore or concentrate to a neutral or partial roast in a fluid bed or multi-hearth roaster, resulting in the conversion of arsenic to a gaseous oxide and/or sulfide. These gaseous arsenic compounds are collected in the off-gas system and stabilized in various forms.
After its removal from the ore or concentrate, fixation of the arsenic into a stable form is critical due to the serious environmental and health issues that arsenic contamination can cause. As disclosed in U.S. Pat. No. 4,126,425, arsenic can be recovered as solid arsenic oxide through sublimation of gaseous arsenic oxide from roaster off-gases. However, due to the poor environmental stability of arsenic oxide and its limited market, it is desirable to fix the arsenic in a more stable form.
There are several disadvantages of conventional processes for arsenic fixation. For example, fixation of arsenic as a stable, crystalline arsenate such as scorodite typically requires expensive oxidation reagents and the need for high pressure autoclaves. Other options, such as the formation of stable ferrihydrites, require high iron to arsenic ratios and the need to limit the arsenic concentration in the final effluent. As a result, arsenic waste streams are often voluminous and must either be discharged to tailing ponds or treated as hazardous waste.
A process has recently been proposed by Adham et al. for direct fixation of arsenic as a stable arsenate from a de-arsenifying roast (Two-Stage Fluid Bed Reactor for Arsenic Removal and Fixation—COM Proceedings, 2014). This process uses a two-stage high-temperature fluid bed reactor, in which the first stage removes the arsenic through a neutral roast, by volatilization as mostly sulfide species. The second stage captures the arsenic from the gas phase through oxidative fixation, as a stable iron arsenate, by reaction with an appropriate iron source.
There remains a need for simpler processes and reactors for fixation of arsenic into stable forms which provide lower equipment costs and/or lower operating costs than known processes and reactors.